Make the primary name of the OF console device /dev/ofwcons, and only
alias it to the contents of the output property if it is defined. This
avoids a panic when booting machines (QEMU) where the output-device
property is not defined.

Since output-device is free-form and potentially conflicts with other
entries in /dev, I also am not sure we should be doing the aliasing at
all, but this at least makes things work again.

Approved by: re (kib)